Various metallic products and parts in the forms of castings, forgings, machine components and the like of ferrous as well as non-ferrous compositions are tested for flaws, voids and similar defects which weaken the metal so as potentially to cause failure in its use. A customary practice is to apply to the metal a liquid penetrant which enters the defect and thus enables visual identification of its character including dimension, extent and location. Commonly used today are penetrants composed of fluorescent dyes which become more plainly visible when exposed to ultraviolet light.